It is certainly possible he damaged some component then, he could have used a crappy PSU, not taken care to avoid static discharge, did not correctly apply thermal paste, etc... I understand it is too late, but to avoid liability on issues like this, you could CYA by taking pictures or video or something before/while you disassemble and package the product. Personally it does not sound like it is your fault, and this seems like a lot to take but it could save future headaches. I am sure as an individual you did not provide any kind of warranty but if you sold it through a site they may have some kind of policy, it just makes it easier when you can say/show "This is how it was working before I disassembled, it was taken apart carefully, and properly packaged, beyond this it is either you (the buyer) or the shipper who is responsible".